A fuel cell is a device used to generate electricity by the chemical reaction of hydrogen gas or other suitable hydrocarbons. A fuel cell generally consists of an electrolyte sandwiched between two electrodes. During operation, hydrogen or other forms of fuel passes over one electrode (the anode), and oxygen or air passes over the other electrode (the cathode) to produce electricity and water and heat as the byproducts. At a molecular level a catalyst at the anode splits an impinging fuel into a positively charged ion and an electron, each of which take different paths to the cathode. When the fuel is hydrogen, the hydrogen atom splits into a proton and an electron. The protons pass through the electrolyte to the cathode, while the electrons are collected to form a current that can be utilized, as electrical power, before they return to the cathode via an external means. At the cathode the protons, electrons, and oxygen are combined, with the aid of a catalyst, to form water. Heat and water are the only byproducts of this chemical process that need to be removed from the fuel cell.
Common types of fuel cell are: Phosphoric Acid Fuel Cells (PAFC), Molten Carbonate Fuel Cells (MCFC), Alkaline Fuel Cells (AFC), Proton Exchange Membrane Fuel Cells (PEMFC), Direct Methanol Fuel Cell (DMFC) and Solid Oxide Fuel Cells (SOFC). Fuel cells may operate at lower temperatures (about 250° C. or less), or at higher temperatures (about 500° C. or greater) depending on their specific type. Lower operating temperature fuel cells include PAFC, AFC, and PEMFC.
Although generally each individual unit fuel cell comprises a membrane (electrolyte) assembly and catalysts interposed between electrically conductive current collector plates, in actual operation mode, multiple unit cells are arranged in series to form a fuel cell stack to meet voltage and/or power requirements. When the individual cells are arranged in series to form a fuel cell stack, the current collector plates are generally referred to as bipolar collector plates, flow-field plates or collector-separator plates. In such an arrangement, the bipolar plate is generally a single plate with one side acting as the cathode and the other side acting as the anode, where each side acts separately as a collector plate (as a cathode or anode) of two neighboring units. In some cases, the bipolar plate can be thought of as comprising different components including a separator plate, which is sandwiched between the cathode and the anode side belonging to the neighboring unit cells. In such a device the separator plates acts as a boundary from one cell to the next cell. There are different variations of the collector plates. For example, the current collector acting as the cathode or anode can be a single plate. Another example is that the current collector, acting as the cathode or anode, and the separator can be a single plate. For simplicity, such plates will be called the collector plates in this invention, which can be the current transferor, separator, cathode, anode, end plate, or any combination thereof.
Regardless of the ultimate form of the collector plates, such plates perform multiple functions that are critical for the entire operation of the fuel cell. First, the collector plates provide a structural support and electrical connection between unit cells. In the case of a single unit fuel cell, the collector plate (which is also the end plate) is connected electrically to an electrical load. Second, the collector plates direct and distribute fuel, and/or oxidant reactants, and/or coolant to, away from, and within unit cells. Third, the collector plates remove products from unit cells and separate fuel and oxidant gas streams between electrically connected cells. In addition to being electrically conductive, collector plates must have good mechanical strength, high resistance to degradation caused by chemical attack and/or hydrolysis, and low permeability to hydrogen gas.
Typically, collector plates have intricate functional patterns formed on the majority of its surface area. For example, complex channels with different patterns, sizes, and shapes, are needed for directing fuel, oxidant, coolant, and byproducts through the fuel cell. The design of the complex patterns depends greatly on the desired pressure drop, resident time, and flow rate. A single channel design would increase pressure drop and resident time, but decrease the flow rate. A multiple channel design would decrease the pressure drop and resident time, but increase the flow rate. The typical dimensions, the depth, and the width of such surface features are on the order of 1 mm, although these features can be substantially smaller than 1.0 mm or substantially larger than 1.0 for special fuel cells. However, such surface intricacies cause significant manufacturing problems for materials commonly used for collector plates.
For example, graphite structures have been traditionally machined to a desired configuration from graphite composite blanks, which is very expensive and time consuming due to the nature of such machining. Although polymer based materials have some advantage with regard to the ease of manufacturing, the collector plates made of polymer based materials are typically inadequate due to the poor electrical conductivity and low strength of the material, particularly with regard to withstanding the compression force necessary to hold multiple fuel cell unit together in stacking applications. Conventional metals and alloys have also been used in fuel cell, but all suffer significant deficiencies. For example, using machined ordinary alloys to produce the necessary detailed surface features has proved to be very expensive. Furthermore, although the lower cost ordinary alloys may be coated with a corrosion resistance layer, which may temporarily solve corrosion problems, such coatings do not represent a satisfactory solution for the long-term stability of the fuel cell structure.
Other issues also arise in the use of ordinary alloys. For example, collector plates should have a high tolerance in the flatness and surface finish of the plate in order to provide an effective seal for the transport of the fuel and byproducts in the gas and liquid form. Any leakage of these gas and liquids, especially in stacked units is not acceptable, and is a critical factor in determining the long-term stability of the fuel cell. For example, typical alloys are readily prone to permanent deformation, such as nicking and denting during fabrication and assembly, whereas graphite plates are extremely fragile without showing any flexibility. Considering the large surface area, high flatness, and small thickness needed in most collector plate applications, the problems of permanent bending and denting for metallic alloys, and the fragility of graphite base materials become severe deficiencies in producing satisfactorily performing collector plates.
Accordingly, a need exists for improved materials for collector plates and collector plates made of such materials.